All Nucleic Acids Move From A 5' to 3' Direction
- Nucleotides link together by phosphodiester bonds between the phosphate of one nucleotide and the 3′ hydroxyl group of the sugar in the next nucleotide.
- As a result:
- The 5′ end has a free phosphate group attached to the 5′ carbon of the sugar.
- The 3′ end has a free hydroxyl (–OH) group attached to the 3′ carbon of the sugar.
Note
- 5′ end is the end of a nucleic acid strand with a free phosphate group at the 5′ carbon of the sugar.
3′ end is the end of a nucleic acid strand with a free hydroxyl group on the 3′ carbon of the sugar.
Significance For Replication, Transcription, and Translation
- It is crucial for you to recognize that DNA synthesis always occurs in the 5'-to-3' direction.
- This means:
- Nucleotides Are Added to the 3' End: DNA polymerase, the enzyme responsible for DNA synthesis, can only add new nucleotides to the 3' hydroxyl (-OH) group of the growing strand.
- Directionality of DNA: DNA strands have a direction based on the sugar-phosphate backbone:
- 5' end: Contains a free phosphate group.
- 3' end: Contains a free hydroxyl group.
- Template and Complementary Strand: The template strand is read in the 3'-to-5' direction, so the new strand is synthesized in the 5'-to-3' direction.
Exam technique
- In Paper 2, questions often ask students to “explain why nucleic acids are replicated in the 5′ → 3′ direction.”
- Always mention:
- Polymerases add nucleotides to the 3′ end.
- Directionality is determined by sugar–phosphate bonding.
Self review
- What structural feature of nucleotides determines the 5′ and 3′ ends of a nucleic acid strand?
- Why must DNA replication always occur in the 5′ → 3′ direction?
